1. Field of the Invention
The invention concerns an optical device for radiation with a wavelength xe2x89xa6160 nm, preferably EUV radiation, with a mirror with a mirror surface, as well as a device for the generation of elastic oscillations of the mirror surface based on surface deformations, as well as the use of such a component, a process for illuminating a predetermined angular range by a light source with such a component, as well as a process for the production of such an optical device.
2. Description of the Related Art
In components for optics, particularly lithography with wavelengths xe2x89xa6160 nm, for example in the EUV range, the problem arises in that, on the one hand, in order to avoid scattering outside the image field or for producing high reflectivities, extremely small residual roughness of 0.1 to 0.5 nm in the HSFR (high spatial frequency range), which lies in the range  greater than 1000 mmxe2x88x921, are required, while, on the other hand, for some applications, a relatively high roughness (1 nm-100 nm) in the MSFR (middle spatial frequency range), which lies between 1000 mmxe2x88x921 and 1 mmxe2x88x921, is required. A component with such a surface specification, for example, is a diffuser for optics with wavelengths xe2x89xa6160 nm, preferably for use in the EUV range.
Such surface specifications cannot be achieved with a polishing process for static components.
One object of the invention is therefore to provide an optical device, for optics using wavelengths xe2x89xa6160 nm, to fulfill these surface specifications.
According to the invention, this object is solved by an active optical device with a mirror comprising a mirror surface, a device for generating elastic oscillations of the mirror surface based on surface deformations, wherein the device is characterized in that radiation impinging on the outer mirror surface, i.e., exterior mirror surface, is diffracted at a predetermined angular range.
An acoustic-optical modulator/deflector has been made known from U.S. Pat. No. 4,346,965, in which a light beam is introduced into a substrate at such an angle that complete internal reflection occurs at an inner substrate surface. The inner substrate surface in turn is configured as the active substrate surface, on whose surface are formed surface waves, so that the radiation impinging on the substrate surface is reflected or is diffracted in the first order. Since light with a wavelength of xe2x89xa6160 nm practically can no longer be transmitted through a solid, but on the other hand, according to U.S. Pat. No. 4,346,965, light must penetrate the substrate surface, the device known from U.S. Pat. No. 4,346,965 is not suitable for light with a wavelength xe2x89xa6160 nm.
An active optical component for EUV lithography, i.e., wavelengths starting at 13 nm, has been made known from U.S. Pat. No. 5,825,844. A component according to U.S. Pat. No. 5,825,844 involves an optical device with a mirror with a mirror surface, wherein oscillations of the mirror surface can be induced by surface deformations. Of course, in the device according to U.S. Pat. No. 5,825,844, radiation impinging on the surface will only be reflected, that is, the angle of incidence and the angle of reflection are identical. The maximum angle of deflection is then limited to xc2x12.5 mrad.
The device of U.S. Pat. No. 5,825,844 is further limited by the fact that possible macroscopic damages of the mirror surface, for example dust particles or scratches, are averaged over time by the active induction of oscillations.
The inventors have recognized that an essentially larger angular range of xc2x112 mrad, for example, can be passed, if, instead of reflection, diffraction is produced at the exterior side of the mirror surface. It is also possible to obtain a completely homogeneous illumination of the angular range of xc2x112 mrad averaged over time by diffraction at grids generated by surface waves, if the frequency of the surface waves is continually varied. In contrast to reflection, in diffraction, light is utilized, which falls in the 1 and xe2x88x921 orders of diffraction of the grid produced by the surface wave and its angle of reflection is thus different from the angle of incidence. In contrast to reflection, in which the incident light thus impinges on the mirror surface so that it illuminates less than a wavelength of the acoustic grid produced by the surface wave, in the case of diffraction, a large number of grid lines, for example, more than 100 grid lines, are impinged.
The wavelength of the acoustical surface wave preferably lies in the range of 1 xcexcm to 50 xcexcm and the amplitude in the range of 1 nm to 100 nm. With such values, one can achieve a ratio of amplitude to wavelength of 1:400 and thus diffraction angles of up to xc2x112 mrad.
In a preferred embodiment of the invention, it is provided that the frequency of the surface waves or surface acoustic waves (SAW) is continuously varied, which is equivalent to a continuous variation of the grid period, so that all angles of diffraction of a source can be adjusted from 0 mrad, for example, to xc2x112 mrad.
The wavelength of the acoustical surface wave preferably lies in the range of 1 xcexcm to 50 xcexcm and the amplitude in the range of 1 nm to 100 nm. Diffraction angles of up to xc2x112 mrad in EUV can be achieved with such values.
For the modification of the incident beam in more than one dimension, for example, in two dimensions, it is advantageous if devices for producing elastic oscillations are provided at several positions on the mirror surface, so that acoustic surface waves can be generated in parallel or not in parallel to the active mirror surface.
If the size of the active mirror surface is insufficient, for example, due to the limited size of piezocrystals, then it is proposed in a further developed form of embodiment to combine the active mirror surfaces of several individual components in order to enlarge the active mirror surface.
In a first embodiment of the invention, it is provided that the device for generating elastic oscillations of the mirror surface comprises a piezoelectric foil. It is preferred that the piezoelectric foil is introduced on the rough side of a sufficiently thin substrate, whereby the other side of the substrate forms the mirror surface for the incident radiation.
The piezoelectric foil can be formed, for example, by a PZT film as is disclosed in VDI-NACHRICHTEN of Dec. 4, 1989, whose disclosure contents will be fully included in the present application. The substrate, onto which the piezoelectric foil is introduced, is a thin Si substrate, for example, a Si wafer. The piezoelectric foil is preferably introduced by means of atomic interconnection techniques, for example, a TiPt connection.
Alternatively to the configuration of the invention with a piezoelectric foil introduced onto a substrate, it is also possible that the total piezoelectric element is comprised of a piezoelectric single crystal.
Point-like or line-form electrodes are used for generating elastic oscillations of the mirror surface on the piezoelectric elements in a first configuration of the invention.
An optical component pursuant to the invention can find use particularly in an exposure device for microlithography with wavelengths of xe2x89xa6160 nm, particularly in EUV lithography.
Possible components in such an exposure device are, for example, means for broadening the beam of the light source, so-called diffusers, means for variation of the illumination setting as well as means for homogenization of the illumination of the pupil.
In addition to the device, the invention also makes available a process for the illumination of a pregiven range of angles by a light source with a wavelength xe2x89xa6160 nm, preferably an EUV light source, with an optical device according to the invention. The process first comprises the step that at least one surface wave of predetermined amplitude and frequency is produced on the mirror surface, whereby the frequency is selected such that the incident light from the light source impinges a large number of grid lines generated by the surface wave. Then the frequency of the surface wave is continually varied in a predetermined frequency range, so that a homogeneous illumination is generated in a pregiven range of angles averaged over time, wherein the range of angles lies preferably between xe2x88x9212.0 mrad xe2x89xa6xcex1xe2x89xa612.0 mrad.
In a particularly preferred process of production of an optical component according to the invention, one side of a piezoelectric substrate is first superpolished, so that the surface roughness amounts to less than 0.5 nm. Electrodes for inducing acoustic surface waves are applied onto the substrate surface, preferably onto the superpolished surface. In an alternative embodiment, a piezoelectric foil can be applied on the rough side of the substrate, and the foil is combined with the rough side of the substrate by means of atomic techniques, whereby the electrodes for inducing acoustical surface waves are attached to the piezoelectric foil.
In another production process, one side of a substrate is first superpolished, so that the surface roughness amounts to less than 0.5 nm. Then electrodes for inducing acoustical surface waves are applied onto the piezoelectric foil.